1. Field of the Invention
This invention relates to a variable capacitor so arranged to precisely control the capacity variation in a wide range.
2. Description of the Prior Art
Conventionally, a PN junction element as shown in FIG. 1 has generally been used as a variable capacitor. In the same figure, the reference numeral 1 denotes an N-type semiconductor region, 2 a P-type semiconductor region, 3 a PN junction, 4 and 5 ohmic electrodes provided in the regions 1 and 2, respectively, 6 and 7 being lead terminals provided to the electrodes 4 and 5, respectively, and 8 represents a depletion layer.
With this arrangement, the depletion layer 8 grows and shrinks in response to bias voltage applied to the lead terminals 6 and 7, and the capacity variation thus caused is read out between them.
However, the conventional variable capacitor adopting such a PN junction element has the following drawbacks:
(1) Due to the fact that the conventional variable capacitor makes use of the increase or decrease of the depletion layer at the PN junction being dependent or the bias voltage, the minimum capacity is determined by impurity concentration in the semiconductor regions while the maximum usable capacity is determined by increase of conductance component. Therefore, it is practically impossible to permit a large variable range of the capacity and hold a large Q factor. Additionally, the larger the capacity variation the higher the Q factor becomes. Therefore, the coventional variable capacitor is attended by difficulties in designing the circuit.
(2) Due to the fact that supply of the bias voltage for varying the capacity and the reading of the capacity variation are performed by means of the common lead terminals, the capacitor is apt to cause undesired capacity variation in response to the input signal itself when the capacitor is adopted in a resonance circuit, for example, resulting in signal deterioration. Further, since it is is usually required that interference between the input signal voltage and the bias voltage be small, the conventional variable capacitor is restricted to a few uses.
(3) The impurity concentration in the semiconductor regions for determining the capacity of the depletion layer is set by such means as diffusion, ion implantation; etc. However, since such means cannot realize a good yield, integration in an integrated circuit is practically impossible.